Powder-metallurgical production of



vent such as alcohol or trichlorethylene.

United States Patent of Sweden No Drawing. Filed May 7, 1962, Ser. No. 12,952 Claims priority, application Sweden May 19, 1961 Claims. (Cl. 75-200) My United States Patent No. 2,920,958 describes the advantageous eifect of the presence of sigma-phase on the sintering of bodies composed of compressed powdered metal. The sigma-phase in fact makes it possible to achieve denser, more pore-free bodies than is otherwise possible under the same sintering conditions. It is presupposed in said patent that the powdered metal has been pressed, before sintering, under high pressure into moulded bodies of comparatively high density, that is to say, of low porosity. The contraction of the body which takes place because of the reduction of porosity during sintering will, therefore, be comparatively small (with the help of certain admixtures it is even possible to get the body to expand during sintering), and it is this which makes it possible, by means of the technique described, to produce sintered bodies of high standards of accuracy.

It has now, surprisingly, been shown that the positive effect of the sigma-phase on the sintering process is so great that it is possible without prior compression to sinter a powder mixture containing sigma-phase to a body having such a high density that it can be use for further treatment, especially heat-treatment, such as rolling and hammering. It is evident that a powerful contraction ensues when a loose powder of comparatively low volume weight is sintered to form a body of considerably higher density. The direct sintering according to this invention does not therefore admit the production of bodies having a high degree of accuracy. But this is not necessary when it is a matter of producing bodies which shall be further shaped later on. It is possible, however, to predetermine to a relatively high degree the size and shape of the sintered body. It is possible thus by the technique according to the invention to produce, for example, tubular bodies for further treatment to tubes, L- shaped or T-shaped bodies for rolling to the corresponding profiles, etc. One can produce sheet bars which are so thin that they can be directly rolled to form a finished product, leaving out the medium and thick rolling stages.

, Prior to the sintering the powder mixture can be shaped as desired by mixing it with a liquid to form a paste which is shaped in the way desired. The paste can for example be extruded through a nozzle to form a cylinder or a tube. The liquid can for example be an organic sol- It must not adversely aifect the particles of powder.

The favourable eifect of the sigma-phase according to the invention is thus made use of in the sintering process. However, the brittle sigma-phase is not wanted in the sintered body or in the object produced from this body. According to the invention, the powder mixture shall, therefore, contain a second metallic component which alloys with the component containing the sigma-phase so that the sigma-phase in the main completely disappears. Metals of the iron group, i.e. iron, cobalt and nickel, used either singly, or in pairs or all three, are especially suitable to be used as the second component. Even metals outside the iron group, such as copper, may be used for this purpose. No general figures of the amounts of this second powder in the powder mixture can be given; it is within the skill of the expert to choose in each particular case the amount which is big enough to make the sigmaphase substantially completely disappear during the sinice 2 tering, that is to say, the powder containing sigma-phase shall be changed by the diffusion of the second component so that the sigma-phase can no longer exit.

In this application I consider that the term sigmaphase does not mean only the pure sigma-phase, but also certain closely-related phases, such as mu, chi-, and xiphases. The terminology in the field is uncertain; certain writers call all these phases sigma-phase, while others diiferentiate between these closely-related phases.

The sigma-phase appears in many different alloy systems. In my patent mentioned at the beginning I have, as examples only, given a number of binary and ternary alloys in which the sigma-phase appears. Sigma-phase often appears in alloy systems containing chromium, as for example in the system iron-chromium-nickel and this makes the invention suitable for the production of subjects of so-called stainless steel.

The sigma-phase is brittle and crushable and the component containing sigma-phase in the powder according to the invention can therefore easily be produced by crushing an alloy containing sigma-phase which has been produced via a molten phase, for example, as is described in United- States Patent No. 2,834,666. If desired, the main quantity of the powder may consist of a powder containing sigma-phase, the other component only needing to be present in such an amount as is required to make the sigma-phase disappear on sintering. In order to obtain a satisfactory density in the sintered body it is, however, sufficient if only a small part of the powder consists of a component containing sigma-phase. In certain cases it appears to be sufiicient to have a content of about 20% of the component containing sigma-phase, at least if its sigma-phase content is fairly high. I prefer to work with a sigma-phase content of more than 50% of the component mentioned, and a content of sigmaphase is preferred, with my preference being formthis x component to comprise at least 50% of the powder mixture.

The sintering to form a body having low porosity and good chemical homogeneity is facilitated if'the powders in the powder mixture are fine grained. It is preferable if the size of grain is less than 0.074 mm. (200 mesh) and I prefer a grain size less than 0.044 mm. (325 mesh). A process of agglomeration with the intention of increasing the fluidity of these fine grained powders, such as is mentioned in my patent mentioned in the introduction, need not in general be resorted to in the manufacture according to the attached invention.

It does not seem to be possible by the method according to the invention to produce a completely pore-free body; at least this would demand such a high temperature and such a long sintering time, that such a process would be practically impossible. One has, therefore, to accept a certain porosity in the sintered body, which makes it important that the conditions of sintering and the subsequent treatment are such as to ensure that the subject can, during this treatment, be compressed to the required density. The extent to which air can be prevented from penetrating into the pores of the body plays an important role in this connection. When sintering begins the pores in the loose powder build a connected pore system, into which the surrounding gases can easily penetrate. The sintering is carried out, as is always the case with the powder metallurgical technique, in a protective gas, usually hydrogen, which thus fills up the pores in the powder, or in vacuum. When the powder contracts during sintering, an increasing number of closed pores, filled with the protective gas and into which air cannot penetrate, are formed by the connected pore system. It is preferable to continue the sintering so long that the remaining porosity substantially consists of closed pores. Owing to the fact that these pores are filled with protective gas their walls Patented Mar. 24, 1964 are reactive and are easily welded together during the subsequent treatment. It appears as if the great majority of the pores exist as closed pores when the total porosity has dropped to about 10%, for which reason I prefer to continue the sintering to the point where the porosity of the body is less than 10%, for instance 28%. It is preferable during a part of the sintering period to expose the surface of the body to a temperature which is higher than the average temperature required for the carrying out of the sintering. In this way there is formed on the surface of the body a sheath which is more pore-free than the body as a whole and which thus prevents the entry of air into the body. To counter the risk of air entering the pores of the subject it is preferable if the body is subjected to the further treatment as soon as possible after the sintering. It can preferably be transferred from the sintering oven direct to the heat treatment (rolling, forging etc.).

By means of the invention bodies can be produced which offer considerable advantages over the conventional ingots reproduced by the metallurgical smelting process. The structure of the body made in the powder metallurgical method will thus be very uniform; one avoids the inconventient features produced by the dendritic structure of the ingot. One also avoids the risk of cavities, pipes which are common with ingots, and further one can make full use of the body as a whole, whereas with ingots one cannot make use of the upper part, the socalled sink head which is often cut off at an early stage of the heat treatment.

The powder is sintered, according to the invention, in a container of suitable shape, for example, in a ceramic crucible or an ingot mould, preferably of fireproof steel. The protective gas may be introduced into the heating oven in which the sintering takes place. The protective gas may be produced by allowing a hydride to be decomposed near the powder. Thus one can introduce into the bottom of an ingot mould a suitable quantity of a hydride which decomposes at the sintering temperature,

after which the powder to be sintered is stamped or rammed into the mould, the mould is closed by a lid and turned so that the bottom end is uppermost and then placed in a heating oven. The hydrogen gas which is formed by the decomposition of the hydride forces all the air out of the ingot mould and the surplus comes out between the ingot mould and the lid.

The powder can thus easily be stamped or rammed when it is poured into the container; it will, though, he on the whole uncompressed, in contrast to the greatly compressed condition in which it is transferred for the production of pressed blanks which are to be subsequently sintered.

A suitable sintering temperature is 1050 to 1350, preferably 1250 to 1350 C. I usually prefer to sinter at a temperature of about 1300 C.

The sintering time will in general be longer than when sintering pressed details, because the bodies which are sintered according to the invention are, in general, considerably bigger than the pressed and sintered details, and because the crucible or the mould hinders the flow of heat and takes time to become hot itself. When sintering small bodies /2 to two hours sintering time can be sufficient, but for large bodies, having for example a weight of hundreds of kilogrammes, a considerably longer sintering time can be called for.

The sintering time and temperature, of course, together affect the sintering process in a known way and it is a matter for the experts skill to decide both these factors so that the sintered body achieves the desired density.

Example 1 For the production of a 200 grams body of stainless steel of the type 18/ 8/Mo, a mixture was made of 100 grams iron powder (Hogan'ais MHP 300.30) and 100 grams of a brittle alloy powder which was made by crushing and finely grinding an alloy produced by metallurgical smelting and containing 45% chromium, 20% nickel, 5% molybdenum and the rest iron. This alloy powder had a structure of more than 75% sigma-phase. The powder mixture was stamped loosely into a round ceramic tube having an inside diameter of 22 mm., with the powder body having a volume weight of 3.85 kg. dm. The tube and its contents were heated in a horizontal tube oven filled with hydrogen at a temperature of 1300" C. for a period of 1 /2 hours, after which the powder had sintered into a round cylindrical body 18 mm. in diameter and having a specific weight of 7.40, which means a residual porosity of about 67%. For purposes of comparison a pressed and sintered blank was made of the same powder mixture, weighing also about 200 grams, by pressing the powder at a pressure of 7.5 tons/cm thus producing a body having a density of about 6 kg./ dm. and sintering this body at 1310 C. for two hours, which yielded a body having a specific weight of 7.51, meaning a porosity of about 5%.

Example 2 Analogous with Example 1 a stainless body was produced of 40% iron powder and 60% pulverized alloy having the composition:

Percent Cr 44 Ni 17 Mo 10 Cu 3 Rest was iron.

Specific weight in sintered state 7.20.

Example 3 Analagous with Example 1 a stainless body was produced of 50% iron powder and 50% pulverized alloy having the composition:

Percent Cr 50 Ni 12 Rest was iron.

Specific weight in sintered state 7.20.

Example 4 Analogous with Example 1 a body of the same chemical composition as in Example 1 was produced from the following powders containing no sigma-phase:

Percent Electrolytic chromium 22.5 Nickel 10 Ferromolybdenum Mo) 3.1 Iron (MPH 300.30) 64.4

The specific weight in sintered state was about 5.0 which shows that the presence of sigma-phase gives a considerably denser body.

Example 5 Analogous with Example 1 a body was produced of the I following powders containing no sigma-phase:

Percent Ferromolybdenum (40% Mo) 50 Iron (MPH 300.30) 50 Specific weight in sintered state was about 5.6.

The ferromolybdenum in this example and the sigma alloy according to Example 1 had almost the same melting point (about 1430 C.). The presence of sigmaphase thus gives a higher density in the sintered body.

Example 6 contains at least 30%, preferably 35-60% chromium. It also contains nickel, for instance 535%, preferably 30%. It may also contain alloying elements such as molybdenum and tungsten, for instance in an amount of 210%. The powder mixture may be further ground after having been mixed, and is placed in a mould, for instance in a ceramic tube. The mould is heated in vacuum or in a protecting gas, for instance hydrogen, at 1100-1350 C. for such a time, usually at least /2 hour and preferably one or several hours, that the porosity of the sintered body is below 10%, preferably 38%. The specific weight of the sintered body is more than 7.2.

What is claimed is:

1. Method for the powder metallurgical manufacture of bodies adapted for further working which comprises sintering a loose, substantially uncompressed, powder mixture comprising at least 20% by weight of a first metallic component consisting of a chromium-containing ferrous alloy at least 50% of which is sigma phase and a second metallic component selected from the group consisting of iron, cobalt and nickel and mixtures thereof, said sec- 0nd metallic component being capable under the sintering conditions of alloying with said first metallic component and being present in quantity suflicient to destroy said sigma phase, the sintering being carried out at a temperature within the range from 1050 to 1350 C. and being continued until the mixture has a porosity of not more than about 10%.

2. Method as defined in claim 1 in which said first metallic component constitutes at least of the mixture and is at least sigma phase.

3. Method as defined in claim 1 in which the sintering temperature is raised during only a part of the total sintering operations to a temperature substantially higher than the average sintering temperature whereby a relatively denser sheath is formed on the sintered body.

4-. A sintered body produced by the method defined in claim 1.

References Cited in the file of this patent UNITED STATES PATENTS 2,920,958 Bergh Jan. 12, 1960 

1. METHOD FOR THE POWDER METALLURIICAL MANUFACTURE OF BODIES ADAPTED FOR FURTHER WORKING WHICH COMPRISES SINTERING A LOOSE, SUBSTANTIALLY UNCOMPRESSED, POWDER MIXTURE CONPRISING AT LEAST 20% BY WEIGHT OF A FIRST METALLIC COMPONENT CONSISTING OF A CHROMIUM-CONTAINING FERROUS ALLOY AT LEAST 50% OF WHICH IS SIGMA PHASE AND A SECOND METALLIC COMPONENT SELECTED FROM THE GROUP CONSISTING OF IRON, COBALT AND NICKEL AND MIXTURES THEREOF, SAID SECOND METALLIC COMPONENT BENG CAPABLE UNDER THE SINTERING CONDITIONS OF ALLOYING WITH SAID FIRST METALLIC COMPONENT AND BEING PRESENT IN QUANTITY SUFFICIENT TO DESTROY SAID SIGMA PHASE, THE SINTERING BEING CARRIED OUT AT A TEMPERATURE WITHIN THE RANGE FROM 1050 TO 1350*C. AND BEING CONTINUED UNTIL THE MIXTURE HAS A POROSITY OF NOT MORE THAN ABOUT 10%. 